Fusobacteria (gram negative anaerobic fusiform bacilli) have been implicated in periodontal diseases and have been isolated from brain and pulmonary abscesses. We have observed in our laboratory hemagglutination (HA) of untreated sheep red blood cells (SRBC) by a Fusobacterium polymorphum cell sonicate. F. fusiforme, which is thought to be identical to F. polymorphum, did not cause hemagglutination at the same concentration as F. polymorphum sonicates. Studies will be undertaken to observe if this characteristic of hemagglutination of SRBC is limited to F. polymorphum or whether other oral fusobacteria may show this characteristic. As of now, all oral fusobacteria are placed under the genus-species F. nucleatum (Bergey's manual, 8th edition). This observed characteristic of HA may provide a taxonomic separation of these important oral microorganisms. Using F. polymorphum, attempts will be undertaken to isolate and characterize by chemical, physical, and serologic means the antigen or antigens which show this hemagglutination activity. Human gamma globulin preparations from periodontal patients have been observed to inhibit the hemagglutination of SRBC by the F. polymorphum antigen preparation. Development of a hemagglutination inhibition test for serologic study of responses to this organism in disease processes may allow further insight into this host-parasite relationship. Attempts will be made to isolate specific human antibody, characterize it by immunoglobulin class, and study the titers of antibody in 75 random human sera and in sera from 20 periodontal patients with indexed disease.